(The below is for after traps on earlier doors, preferably very magical since this isn't until later. Also, earlier in the tower is found a powerful magnet, perhaps as part of some other trap.)
A steel door at the top is in front of the steps. No magic can be detected on the door. (Note that Detect Magic is blocked by 1 inch of common metals, so the magic animating the construct beyond it won't be detected.) A rogue should be able to tell that the whole door is well crafted and extremely sturdy, especially the hinges.
When opened a load of 4" iron balls that are piled against it to pour our and down, washing the adventurers down the hard steps in a flood of iron balls. Ouch. Allow people to try to do things to get out of the way, dive into other doors, or tough out the balls so you get hit with them but don't also fall down the stairs. Which will have the added benefit of splitting the party.
Oh, if there are pressure activated traps on some stairs and the party has learned how to avoid them, this will set them off. One that does a nasty cloud spell that cuts of vision and is bad to go through would be great.
Then a construct attacks from in the room. BUT at the same time, all of the enchanted balls at the bottom also assemble themselves into a [level-appropriate] golem that comes up behind the distracted and split-up party for a battle on two fronts.
The way the heir would get around this is to attach the magnet to the door. The steel balls won't roll down so they won't activate, and they can walk right in after opening the now very heavy but well balanced door on the hinges reinforced to take the full weight.